Due to continuously tightening regulations concerning nitrogen oxide (NOx) emissions of internal combustion engines in ships and power plants, selective catalytic reduction (SCR) systems are becoming more and more common. With the SCR, NOx reductions of up to 90% or even greater can be achieved.
In an SCR system a catalyst material and a reducing agent are used to decompose the NOx formed during the combustion back to the basic elements. The catalyst is arranged on the surface of a supporting ceramic material forming a honeycomb structure inside a reactor. On the surface of the catalyst, the NOx reacts with ammonia that is used as the reducing agent and nitrogen and water is formed. In practice, urea is often used instead of ammonia for safety reasons. This applies especially to the marine applications, where leaking ammonia is a serious hazard. Urea is injected into the exhaust gas flow in a water solution. Due to the high temperature of the exhaust gas, water evaporates and the urea molecules break up into ammonia and carbon dioxide.
Correct dosing of the reducing agent is important for several reasons. If the amount of the reducing agent that is injected into the exhaust system is too small, NOx reduction is not as effective as it could be. On the other hand, excessive dosing of the reducing agent results in ammonia slip, which means that part of the ammonia flows through the SCR system without reacting with the NOx. Ammonia emissions are even more harmful than the NOx emissions, and therefore overdosing of the reducing agent is an undesired situation. If the exhaust system is provided with an oxidation catalyst that is placed down-stream from the SCR elements, at least part of the excess ammonia is oxidized. However, this is not an optimal solution for the reduction of ammonia slip, since NOx is formed in the oxidation process. If the dosing of the reducing agent can be optimized, also the size of the reducing agent tank can be minimized.
To achieve accurate dosing of the reducing agent, an SCR system can be equipped with a NOx sensor that measures the NOx concentration downstream from the SCR elements. Patent application WO 2008043928 A1 discloses an SCR system of a vehicle where a NOx sensor is placed downstream from an SCR catalyst and the NOx and ammonia concentration of the exhaust gas is continuously monitored. According to the method, an initial amount of urea is injected into the exhaust system. If the engine is running at steady state, the urea dosing is increased by multiplying the initial amount by a coefficient greater than one. After increasing the dosing, NOx and ammonia concentration is measured, and if the measurement shows decreased concentration, the dosing is increased again. This step is repeated until the concentration starts to increase. This is an indication of overdosing, and urea dosing is therefore decreased by multiplying the previous amount by a coefficient smaller than 1. This step is repeated as long as the concentration reduces. When the concentration starts to increase again, the urea dosing is increased.
A problem with the control method presented above is that it is not suitable for large internal combustion engines. In large internal combustion engines, such as in the engines used in ships and powerplants, the stabilization of the SCR system takes a relatively long time after the dosing of the reducing agent is changed, and therefore a method comprising several iteration steps is too slow. A problem related to continuous NOx measurement is that the lifetime of a sensor that is continuously exposed to the exhaust gases is very limited.